1. Field of the Invention
This invention is directed to screening material for screens for vibratory separators, to screens made from such material, to vibratory separators with such screens, and to methods of their use.
2. Description of Related Art
Various woven cloth screens for vibratory separators are used in removing particles from a liquid and are designed to provide a tortuous path for the liquid. Many prior art woven cloths, including the typical weave, twill, dutch weave or twill dutch weave cloths have low fluid conductance characteristics due to the formation of the tortuous flow path. A minimal rate of flow results in a correspondingly slow filtering process. Often the screens need frequent cleaning to maintain a desired flow rate.
Certain prior art screens provide an open surface area and permit direct or nontortuous flow through the screen. Such screens may provide better conductance characteristics, but the fluid conductance may be limited by the permissible ratio of length to width in the interstices between the screen filaments and the fineness of the filaments. With increasing spacing between filaments, deformation of the filaments from the parallel may increase and larger than desired particles can then pass through the screen. To maintain an efficient relationship, the size of the rectangular interstices in these screens is generally minimal and the length to width ratio is generally less than three unless coarse, stiff filaments are used. Higher ratios can be achieved by bonding together the crossing filaments of the screen; but bonding can be a complex and costly process with negative side effects. By coating the filaments with a bonding material, the diameter of the filaments is increased, further reducing fluid conductance of the screen.
Particle separation, fluid throughput or conductance and screen life are important characteristics of screens for vibratory separators. Finer particle separation results in a higher percentage of impurities being removed from the screened fluid. Higher conductances are desirable because more fluid can be processed per square foot of screen area, thereby reducing costs. Doubling conductance doubles the liquid throughput. Longer screen life saves time and money. Since the mid-seventies one vibrating screen industry trend has been to decrease wire diameter in order to achieve higher conductance. For certain prior art screens this has means finer separation and higher conductance but shorter screen life. To increase screen life, the industry has tried various types of bonded screens such as plastic-backed, metal-backed or bonded-backup. These bonded screens are relatively expensive. U.S. Pat. Nos. 5,370,797; 5,256,291; and 5,256,292 disclose double shute or warp screens with a double warp plain weave screen having warp and shute wires of the same material and properties, the shute diameter at least 1.4 times the warp diameter to prevent sleaziness. If the shute diameter controls the conductance and if the shute diameter is fine enough to give very high conductance, the warp diameter is so fine that the screen has a low tensile strength and therefore shorter life; and screens for removing undesirable particles from a liquid in which a substantially flat parallel array of shute filaments are spaced at less than a preselected minimal linear dimension of undesirable particles and a parallel array of groups of warp filaments runs transverse to the shute filaments. The warp filaments of each group are oppositely woven about and between the shute filaments taken individually or in pairs to secure the shute filaments and maintain the spaces therebetween. The groups of shute filaments have spaces therebetween smaller than the preselected minimal linear dimension of the undesirable particles so that the screen is characterized by elongated rectangular flow apertures therethrough. Each group includes from 3 to 10 or more warp filaments and the shute filament diameters are as small as in the order of 1.1 times the warp filament diameter. Conductance is increased by making the rectangular apertures longer. The life of the screen is increased by increasing the number of warp wires to achieve the required tensile strength. Finer particle separation is achieved by making the short dimension of the rectangle smaller. Screens formed by this weaving of groups of three or more warp filaments transverse to shute filaments which are as small as in the order of 1.1 times the diameter of the warp filaments provide meshes having relatively higher aspect ratios with smaller filament diameters than with certain known weaves of filaments of this range of diameter.
FIGS. 1A and 1B show a prior art screen 22 as disclosed in U.S. Pat. No. 2,723,032 with a coarse mesh wire screen, or cloth 23 that provides a backing screen or cloth of the unit. A fine mesh wire screen 24 is superimposed or mounted upon the backing screen 23. The screen unit 22 has its coarse backing wire mesh or cloth coated or covered preferably with rubber or some suitable rubber or synthetic rubber composition. The strands are indicated at 25 and the covering or coating at 26. Since all of the strands 23 are coated or covered, there is, of course, rubber-to-rubber contact between these strands of the coarser mesh screen 23. The backing screen of cloth 23 is of the roller flat-top type and of any coarse size such, for example, as three of four mesh. The mesh of the finer mesh wire screen 24 varies, in accordance with the separating job to be done. For example, the mesh of the fine wire screen or cloth 24 may vary from the order of minus 20 (-20) to the order of minus 325 (-325).
FIGS. 2A and 2B disclose a screen 30 as disclosed in U.S. Pat. No. 4,696,751 with a first mesh screen with rectangular dimensions of width and length. A second screen 38 is held in superimposed abutting relationship to the first screen 32. The second 38 has width and length dimensions. The length dimensions of the first screen is larger than length dimension of the second screen, and the width dimension of the first screen is smaller than the width dimension of the second screen.
FIGS. 3A and 3B disclose screens 50 and 53 shown in U.S. Pat. No. 5,626,234 which has an upper cloth 51 and lower cloth 52. The upper cloth 51 is formed from woven stainless steel wire in the range 0.19 mm to 0.036 mm diameter and 60-325 mesh, (i.e. number of strands per inch) while the lower cloth 52 is formed from woven phosphor bronze wire in the range 0.45 mm to 0.19 mm diameter and 20-40 mesh. A screen 53 in FIG. 3B has an upper cloth 54 like the upper cloth 51 (FIG. 3A) and a lower cloth 55 woven from stainless steel wire having a nominal diameter in the range 0.20 to 0.45 mm diameter and typical 30 mesh, and is coated with an epoxy based material, or Molybdenum Disulphide, or Teflon (Registered Trade Mark), to a thickness in the range 5 to 50 microns typically 20 to 40 microns. Multiple passes of the wire through a coating process or through a succession of such processes may be necessary to achieve the desired coating thickness. The wires 57, 58, 59 are shown in cross section to show the outer material coatings 67, 68, 69 (not to scale). The wire 64 is shown with the coating scraped from one end.
There has long been a need for effective and efficient screening material for screens for vibratory separators, e.g. but not limited to, for shale shakers. There has long been a need for such screens with relatively fine diameter wires, but with desirable throughput, conductance, and non-clogging, non-blinding characteristics. There has long been a need for such screens that have sufficient support for relatively finer mesh screens and are durable in their intended uses.